It is generally accepted that most electronic musical instruments, especially keyboard instruments, produce less satisfying musical results than do the acoustic instruments they attempt to imitate. Thus the electronic organ does not produce music of the same majestic quality as the pipe organ, and most electronic pianos produce results that at best can be described as pale imitations of the real thing. One of the main differences is the relative simplicity of the electronically produced signals, and the lack of the chorus, or in the case of the piano, the multiple unison effect which is inherent in the real instruments. The real instruments almost always use a large number of separately tunable tone sources, more or less widely spaced, and the acoustic wave beats that result when several notes are played simultaneously is the basis of the satisfying chorus sound. A number of attempts have been made to overcome this lack of chorus by electrically processing signals to increase their complexity. Such systems are shown in U.S. Pat. Nos. 3,083,606 and 3,160,695 both issued to Donald L. Bonham. Other patents in the same field include U.S. Pat. No. 3,749,837 issued to James Doughty, and U.S. Pat. No. 3,833,752 issued to Tijmen van der Kooij. The Bonham U.S. Pat. No. 3,083,606 employs a plurality of vibrato units for frequency modulating signals from a musical instrument. Individual vibrato units are cyclically driven in different phase relationships so that the musical signals are instantaneously detuned by different amounts in each of the plurality of different channels. The result is a desirable chorus effect if the vibrato frequency is low (i.e., in the order of 1/2Hz) or a chorus vibrato if the vibrato is in the order of 4 to 7 Hz. Bonham U.S. Pat. No. 3,160,695 accomplishes a similar result by using an audio delay between two amplifiers each of which is coupled to a separate loudspeaker, with the delay accomplishing the phase shifting of the vibrato modulation. The result again is a chorus tremolo effect. The Doughty patent describes a variable digital delay line system arranged to produce vibrato effect. The digital vibrato systems of Doughty are ideally adapted to modern integrated circuit techniques. The "bucket brigade" analog, digital, variable, delay lines as described by Doughty are particularly suitable as the basic vibrato modulators required in the chorus systems of Bonham.
The van der Kooij patent describes a variation of the Bonham system using Doughty type vibrato modulators. The present invention is an improvement on systems of the general type just described.